1. Field of the Invention
The present invention relates to a continuous process for polymerization of 1-alkenes. In this process, the inherent viscosity of the polymer product remains centered around a targeted inherent viscosity value.
2. Background Information
In the area of viscoelastomeric materials, especially pressure sensitive adhesives (PSAs), precise and predictable control over various mechanical and process properties is desirable so that the materials can be tailored for specific, highly demanding end use applications such as packaging tapes, medical tapes, masking tapes, and vibration damping materials. These applications require a proper balance of properties, and this balance changes with each end use.
With the discovery of the catalytic activity of Ziegler-Natta (ZN) coordination systems toward 1-alkenes (i.e., .alpha.-olefin monomers) and the subsequent production of high molecular weight poly(.alpha.-olefins), the elimination of double bonds from the polymer backbone, which had detracted from the oxidative stability of such polymers, became possible. The homopolymers of C.sub.6 to C.sub.10 .alpha.-olefins were naturally tacky, had low toxicity, aged well, and were environmentally stable (i.e., did not oxidize readily), chemically inert, resistant to plasticizer migration, and relatively inexpensive. These characteristics made them good candidates for PSAs. However, their poor cohesive strength meant that they lacked the shear adhesion required of high performance PSAs. Some examples of the preparation of poly(.alpha.-olefin) PSA compositions via ZN catalysis include U.S. Pat. No. 3,542,717 (mixtures of polyolefin copolymers derived from olefin monomers with different molecular weights), U.S. Pat. Nos. 3,954,697 and 4,178,272 (hot-melt adhesives derived from copolymers of propylene and C.sub.6 to C.sub.10 .alpha.-olefins), and U.S. Pat. No. 4,288,358 (compositions containing, inter alia, terpolymers of propylene, 1-butene/1-pentene, and C.sub.6 to C.sub.10 .alpha.-olefins).
More recently, radiation curing of .alpha.-olefin homo- or copolymer compositions (ZN-catalyzed) has been shown to provide PSAs with a good balance of peel and shear properties, and metaliocene catalysts have been identified as catalysts for olefin polymerizations. (These catalysts are capable of producing very stereoregular homopolymers as well as copolymers that can include a high ratio of mer units derived from lower 1-alkenes such as ethylene and propylene.)
The production of poly(.alpha.-olefins) has been described in a wide variety of reaction vessels and polymerization methods including continuous stirred tank reactors (e.g., U.S. Pat. No. 3,074,922), loop reactors (e.g., Zacca et al., "Modeling of the Liquid Phase Polymerization of Olefins in Loop Reactors," Chem. Eng. Sc., 48, no. 22, 3743-65 (1993)), tubular reactors (e.g., U.S. Pat. No. 4,383,093), screw conveyors (e.g., U.S. Pat. No. 2,894,824), extruders (e.g., U.S. Pat. No. 4,058,654), fluid bed reactors (e.g., U.S. Pat. No. 4,482,687), and static mixer reactors, (e.g., U.S. Pat. No. 4,792,595). However, consistent control, over time, of the molecular weight (MW) of the resultant polymer has proven elusive. For example, in the loop reactor polymerization described in the Zacca et al. reference, temperature oscillations are unavoidable. Such oscillations cause the MW of the polymer (as measured by its inherent viscosity) to fluctuate over time. This results in polymer fractions that are undesirably high and low. Control over MW is imperative if the resulting polymer is to be useful as a high performance PSA material.
Chain transfer agents, especially hydrogen because of its cost and availability, have been used to control the MW of polymers including poly(.alpha.-olefins). However, many chain transfer agents are pyrophoric and/or explosive and, therefore, present significant safety and handling challenges. Additionally, control of the MW of the poly(.alpha.-olefins) is complicated by the degree that the chain transfer agent (especially when a gas such as hydrogen is used) is able to mix with the reaction mixture.
That which has not been described previously is a continuous method of producing poly(.alpha.-olefins) that provides consistent control over MW of the resulting polymer without using a chain transfer agent such as hydrogen gas.